1. Field of the Invention
The present invention relates to a process for producing a crosslinked polyethylene resin foam. More particularly, it relates to the production of crosslinked polyethylene resin foam in which the crosslinking is accomplished through contacting the hydrolyzable silyl groups on the side chains of the polyethylene resin, which is a constituent of the foam, with water.
2. Description of the Prior Art
There have been disclosed many kinds of techniques of producing crosslinked polyethylene foams. According to one of them, a mixture of polyethylene regin and blowing agent is irradiated with ionizing radiation so that the crosslinking reaction takes place, and then the mixture is heated under normal pressure so that the expansion takes place. According to another typical technique, a mixture of polyethylene resin, organic peroxide, and thermal decomposition type blowing agent is heated at a temperature lower than the decomposition point of the blowing agent and higher than the decomposition point of the organic peroxide so that the crosslinking reaction takes place, and then the mixture is heated under normal pressure at a temperature higher than the decomposition point of the blowing agent so that the expansion takes place.
These known techniques are being employed for the commercial production of crosslinked polyethylene resin foams, but they are not necessarily satisfactory and have still some disadvantages to be overcome. That is to say, the method involving radiation is disadvantageous in that a special expensive apparatus is required and radiation does not reach deep inside the thick sheets and other thick-walled moldings, with the result that uniform crosslinking is not performed. The method for crosslinking with a peroxide has also a disadvantage that the crosslinking reaction tends to take place prematurely when a polyethylene resin is incorporated with a peroxide with heating and kneading, with the result that it is difficult to produce foamed sheets of uniform thickness.
In addition to the crosslinking methods employing radiations or organic peroxides, there is another known method. According to this method, the polyethylene resin constituting the foam is made crosslinkable by the introduction of hydrolyzable silyl side chains and the crosslinking is accomplished through the hydrolysis of the silyl groups. In other words, a polyethylene resin is copolymerized with an ethylenic unsaturated silane compound by random copolymerization or graft copolymerization. The copolymerized polyethylene resin is then mixed with a mixture of thermal decomposition type blowing agent and crosslinking catalyst. PG,4 The resulting resin composition is formed into a sheet and other molding. The moldings are brought into contact with water to bring about the crosslinking reaction. Thereafter, to accomplish expansion, the moldings are heated under normal pressure above the decomposition point of the blowing agent used.
This method, however, is also as unsatisfactory as the above-mentioned methods, because the step for preparing expandable moldings (e.g., sheet) and the step for expansion are performed separately and the crosslinking cannot be carried out at a sufficiently high temperature.
A problem commonly encountered in the above-mentioned conventional methods is that the resulting foamed product tends to shrink a great extent when exposed to high temperatures for a long time. This is caused by the so-called memory effect which is experienced when expansion is performed after crosslinking.
There is known a method for producing foams (such as an expanded sheet) by performing molding and expansion in one step. According to this method, a thermoplastic resin is mixed with a blowing agent and the resulting mixture is extruded into a low pressure zone where the extrudate is allowed to expand, and the thus prepared foam is crosslinked by the above-mentioned means. This method does not involve the problem of memory effect.
This method, however, suffers from a disadvantage. In actual production, a polyethylene resin having silyl groups, a blowing agent, and a silanol condensing catalyst are mixed in an extruder and the extrudate is allowed to expand upon emergence from the extruder. The expanded extrudate is subsequently brought into contact with water in order to bring about crosslinking. Since contact and absorption of water take a long time, crosslinking also takes a very long time. An attempt to reduce the crosslinking time by introducing water into the extruder have been unsuccessful because crosslinking takes place in the extruder or extrusion die and the crosslinked resin undergoes a high shear which breaks molecules mechanically and generates heat. Introduction of water is undesirable particularly where products having a high degree of crosslinking or a high degree of expansion are to be obtained. This undesirable result is experienced even when water is not introduced intentionally into the extruder. Namely, the water unavoidably enters into the extruder, such as by adhering to the resin, which results in producing the crosslinking reaction, whereby stable extrusion can never be achieved. Also, moisture on the resin pellets could prevent the production of foams of uniform quality.
Furthermore, the above-mentioned method has another disadvantage that the extruded foam tends to shrink and does not recover easily upon ageing in the heated air. Moisture in the air causes additional crosslinking to take place during ageing, with an adverse effect on the recovery by ageing. Incomplete recovery leads to an insufficient degree of expansion. Presumably, this is because crosslinking causes the cell walls of the foam to decrease in gas permeability or the shape of the cell walls to be fixed.
In order to overcome the above-mentioned disadvantages, the present inventors completed this invention.